Graham Nash and David Crosby make a surprise appearance at the Beacon Theatre in New York City for the Tune In to Hep C benefit concert.

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Last summer, Gregg Allman was confined to his Savannah home, recovering from a transplant that became immediately necessary after his doctor found cancer on his liver. “It’s a painful thing,” he told Rolling Stone last night at the Beacon Theatre, where the Allman Brothers Band played a rare, off-tour, three-hour show. “While I was healing, I kept wondering what it’s going to be like a year from now.”

Last night, one year and one month later, he got his answer – with help from friends Natalie Cole (who slayed “Whipping Post”) David Crosby and Graham Nash (who lent their timeless harmonies to several of their classics and rarities), and Phil Lesh (who jammed with the band on a string of Grateful Dead classics). It was that kind of night.

The show was a benefit for Tune in to Hep C, a campaign to raise awareness about the virus, which Allman (who believes he contracted it at a tattoo parlor in his twenties), Lesh (who had a liver transplant in 1998), Crosby and Cole have all been diagnosed with. It affects 3.2 million Americans, including many in the rock & roll community, from Steven Tyler to Keith Richards. Its symptoms can lie dormant for decades, ravaging the liver until it’s too late.

Despite the serious matter, last night was no stuffy benefit. The Allman Brothers’ first set included the groove-heavy “One Way Out,” Gregg Allman in remarkable voice; Warren Haynes’ soulful take on Van Morrison’s “Into the Mystic”; and the chugging bliss of “Statesboro Blues.” Natalie Cole stole the show relatively early on, revving up the crowd adding her cackled soul to “A Change is Gonna Come,” which was so awe-inspiring it ended with bassist Oteil Burbridge bowing to her.

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The band ended the set with the funky, salsa-tinged “In Memory of Elizabeth Reed,” which shifted into another realm, Trucks going on a blissful slide guitar escapade while Haynes stabbed his guitar strings with raucous fury. “We’re just getting started,” he said afterward.

Indeed, the fun had barely begun. After an intermission, Crosby and Nash unsuspectingly appeared without the band to room-shaking applause. “I don’t know about singing folk music after what’s been going on onstage,” Crosby said wryly, “but we’re gonna try.” They performed a gorgeous “Teach Your Children,” with Trucks adding tasteful slide guitar while many in the crowd shouted every word. “Okay,” Crosby said afterward. “If you can do that, I don’t feel so bad.”

The duo then traded haunting, atmospheric harmonies on 1969’s “Guinevere,” with their eyes closed with their voices locked. Soon the Allman’s emerged with Lesh for “Cowboy Movie,” a rarity Crosby introduced as about CSN’s “second or third breakup.” “I haven’t sung it since then,” he added, “because it has a lot of words and I was too stoned to remember then.” It sounded loose and raw, evolving into an epic jam with Haynes and Trucks trading scorching licks. Crosby then delivered a stunning “Almost Cut My Hair,” which had a focused Trucks mimicking the melody on his slide guitar. Crosby grinned, watching every note, appearing in awe of seeing his melody come alive with such intensity.

The duo left while Lesh joined the band for the ultra-funky “Shakedown Street,” and joyous singalongs of “Sugaree,” and “Franklin’s Tower. Seeing these road dogs have so much fun was pure joy, the kind of thing that could’ve happened 40 years ago.

As if it couldn’t get any better, Cole reemerged to howl an apocalyptic “Whipping Post.” Then, all the guests (sans Lesh) emerged for the finale, which included a decked-out Billy Gibbons of ZZ Top emerging for the spiritual “Will The Circle Be Unbroken?” It felt slow and a little sloppy, everyone looking at eachother for cues, but it didn’t matter. After the lights went on, Crosby and Trucks hugged offstage while Gregg Allman ventured to the edge of the stage, grinning and grabbing fans hands. “We call the stage the perfect planet,” Allman said the day before the show. “Once the music starts, you don’t feel any pain up there.”